Electro-optical image device



i 1940- H. G. LUBSZYNSKI ET AL ,185,857

ELECTRO -OPTI CAL IMAGE DEVI CE Filed March 4, 1938 Mil/EN TORS H. a L uBsy/vs/r/ H/IROLD MILL ER Patented Jan. 2, 1940 ITE SATES 2,185,857 7 ELECTED-OPTICAL IMAGE DEVICE Great Britain Application March 4, 1938, Serial No. 193,856 In Great Britain March 10, 1937 4 Claims.

The present invention relates to electron optical systems of the kind in which an electron beam is caused to pass through a magnetic field which acts as a lens in respect of the beam.

The object of the invention is to provide an arrangement for setting up .a magnetic lens field. of which the magnification or power may be readily varied as desired preferably by means located outside the evacuated envelope of the electron discharge device, in association with which the lens is arranged.

According to the present invention an electron optical system in which an electron beam is caused to pass through a magnetic field which acts as a lens in respect of the beam, is provided, said optical system including an apertured member of magnetic material so arranged in conjunction with a coil or magnet system by which the magnetic field is set up as to modify the power of the lens in respect of the traversing beam. In carrying the invention into practice the apertured member of magnetic material may be arranged outside the envelope of the -device including the electron optical system between the lens coil or magnet system and a utilising element on to which the electron beam is projected, in which case the efiect of the said member is to increase the magnification produced by the lens. Alternatively the apertured member may be arranged between the source of electrons from which the traversing beam emanates and the lens coil or magnet system, in which case the said member causes a reduction of magnification. If desired, two apertured members may be provided arranged one on each side of the lens coil or magnet system.

In an arrangement according to the invention the position of the apertured magnetic shield may be adjusted and/or the size of the aperture in the apertured magnetic shield be varied to vary the magnification oi the electron optical image produced on the image utilising element. If desired the aperture in the apertured magnetic shield may be non-circular in shape whereby the efiect of the focussing magnetic field in combination with the shield on the electronic rays resembles that of a non-spherical optical lens on light rays. In order that the electron image shall remain in focus on the utilizing element after the position of or the size of the aperture in the magnetic shield has been adjusted, it is necessary to vary the strength of the magnetic field constituting the lens, for example, by varying the current through the lens coil in order graph showing the effect of the shield in the A arrangement of Fig. l on the field distribution along the axis of the focusing coil.

In Fig. 1, C is a source of electrons such as a photoelectric cathode on which an optical image may be projected by a suitable optical system (not shown), and S is a fiuorescent screen on which electrons leaving cathode C are to be focused.

The cathode C and screen S are housed in a suitable evacuated envelope not shown. Outside the envelope, arranged near the cathode C, is a magnetising coil M, which produces a field which is axially symmetrical. The coil M is preferably of large diameter so that the spiral distortion which is usually produced at the edge of an electron optical pictureby a focusing coil is small. For example, in a particular case the photocathode had a diameter of six centimetres and the magnetising coil a diameter of eighteen centimetres. With the plane of the coil l1 substantially in the plane of the cathode C it was found that a magnification of the image projected on the cathode of 2:1, could be obtained on the screen S with good definition and very little distortion of the magnified image. Now according to the invention, the magnification obtainable in this arrangement is increased by providing between the coil M and the screen S an apertured iron plate P surrounding the evacuated housing in which the cathode C and screen S are located. For example, using a sheet having a central circular aperture of diameter eight centimetres, the sheet being'located in a plane 5 centimetres away from the central plane of the coil M, a magnificationof theimage projected on the cathode of .3:1 was obtainable, the twist of the image being 20 and with the shield or plate P closer to the coil M a magnification of 4:1- was obtainable, with 'even less twist of the image thanin the former case. I

An alternative method of varying the magnification is to keep the shield in a fixed position and to vary the diameter of the aperture, for example by means of an iron iris diaphragm.

It is, of course, not essential for the coil M and v a magnification of 4:1.

plate P to be arranged on the outside of the evacuated envelope including the cathode C and screen S as described above, but with this arrangement, the advantage is obtained that the position or aperture of the shield P may be readily adjusted.

The efiect of the plate or shield P on the field distribution along the axis of the coil may be seen from examination of Fig. 2 of the drawing in which distance along theaxis of the coil measured from the centre of the coil is represented horizontally and field strength vertically. The arrow P indicates the position of the shield P.

The curve A represents the field distribution without the shield P in position and the curve B shows the field distribution along the axis of coil M with the shield P located as aforesaid at a distance of 5 cm. from the central plane of the coil. It will be observed by comparing these two curves that the field on the axis in the neighbourhood of the plane of the coil is increased by some twelve percent, but outside the shield the effective field strength is decreased. Thus the principal plane of the lens will move nearer to the cathode when the shield P is introduced.

Since the power of the lens is proportional to where Hz is the field strength in the axial direction Z, the power of the lens with the shield in'place will be seen to be increased somewhat and a corresponding reduction in the current in the coil M will be necessary if the electron image is to be focused in the same plane as before without moving any of the components of the optical system.

If desired the opening in the magnetic shield 'P may be made other than circular in shape.

For example, the opening may takethe form of a rectangular slot, so that the shield would then produce an effect on the electron rays analogous to that of a cylindrical optical lens on light rays.

Thus, in a particular case, a magnetising coil of which the'diameter was 14 cm. was found to produce with a shield arranged at a short distance away from it having a circular opening seven centimetres in diameter a magnification in the electron image of 7 :1, the coil alone giving When the apertured shield was replaced by a composite shield formed of two members about twenty centimetres wide with parallel edges defining a slot seven centimetres wide it was found that the magnification of the image in the plane perpendicular to the length of the slot was 10:1 whilst the magnification in the central plane extending lengthwise of the slot was 2: 1, the ratio between the transverse and longitudinal magnifications thus being 5:1. When the width of the slot was'increased to nine centimetres the transverse magnification ratio decreased to 8:1 and the longitudinal magnification rose to 3:1. The effect of widening the slot is thus to cause the apertured shield to operate more nearly as though the aperture were circular.

If desired the form of the aperture in the shield P may be such as to compensate for nonspherical aberration or distortion produced in the electron optical system.

In the arrangement represented in the drawing the apertured plate P is shown as located on the side of the lens coil M remote from the cathode C. However, if desired, the plate P might be arranged on the same side of the coil M as the cathode C, in which case the plate P would be effective to reduce the magnification of the lens M. Moreover if desired, plates such as P might be provided on either side of the coil M so that the magnification thereof might be increased or decreased as required according to the relative position of the plates and the relative sizes of their apertures.

We claim:

1. In an electron device comprising a photoelectric cathode of predetermined diameter, an electron utilizing target in register with said cathode, a magnetic coil having a diameter at least twice the predetermined diameter of said cathode, said coil being positioned symmetrical with and intermediate the cathode and target, and a planar magnetic shield adapted to be moved parallel to the plane within which the coil lies for controlling the focusing and twist of electrons emanating from thecathode and passing through the coil.

2. In an'electron device comprising a photoelectric cathode of predetermined diameter, an electron utilizing target in register with said cathode, a magnetic coil having a diameter at least twice the predetermined diameter of said cathode, said coil being positioned symmetrical with and intermediate the cathode and target, and a planar magnetic shield between said coil and screen, said shield being adapted to be moved parallel to the plane within which the coil lies for controlling the focusing and twist of electrons emanating from the cathode and passing through the coil.

3. In an electron device comprising a photoelectric cathode of predetermined diameter, an electron utilizing target in register with said cathode, a magnetic coil having a diameter at least twice the predetermined diameter of said cathode, said coil being positioned symmetrical with and intermediate the cathode and target, and a planar magnetic shield between said cathode and coil, said shield being adapted to be moved parallel to the plane within which the coil lies for controlling the focusing and twist of electrons emanating from the cathode and passing through the coil.

4. In an electron device comprising a photoelectric cathode of predetermined diameter, an electron utilizing target in register with said cathode, a magnetic coil having a diameter at least twice the predetermined diameter of said cathode, said coil being positioned symmetrical with and intermediate the cathode and target, and a planar symmetrically apertured magnetic shield between said cathode and coil, said shield being adapted to be moved parallel to the plane within which the coil lies for controlling the focusing and twist of electrons emanating from the cathode and passing through the coil.

HANS GERHARD LUBSZYNSKI. HAROLD MILLER. 

